Add --with-openssl-engine autoconf option (auto|yes|no)

Add --with-openssl-engine autoconf option (auto|yes|no)

This allows to select engine support at configure time. For OpenSSL 1.1 the
default is not changed and we detect if engine support is available.

Engine support is deprecated in OpenSSL 3.0 and for OpenSSL 3.0 the default
is to disable engine support as engine support is deprecated and generates
compiler warnings which in turn also break -Werror.

By using --with-openssl-engine=no or --with-openssl-engine=yes engine
support can be forced on or off. If it is enabled but not detected an
error will be thown.

This commit cleans up the configure logic a bit and removes the
ENGINE_cleanup checks as we can just assume that it will be also
available as macro or function if the other engine functions are
available. Before the cleanup we would only check for the existance
of engine.h if ENGINE_cleanup was not found.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Max Fillinger <maximilian.fillinger@foxcrypto.com>
Message-Id: <20211019183127.614175-3-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23000.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>